A. Technical Field
The present invention relates to a bio-based coating composition and its coated article. In detail, the present invention relates to: a coating composition based on lactic acid that is a bio-based ingredient; and an article coated with the coating composition.
B. Background Art
At present, plastic substrates are widely used as household goods and industries, and its production amount is vast. Most of such plastic substrates are disposed of as waste after use. However, depending on how to dispose of the waste plastic substrates, they may have bad influences on the global environment.
For example, in cases where incineration disposal is carried out, vain energy becomes necessary for the incineration. In addition, there is a worry that the global warming may proceed due to carbon dioxide generated by the incineration, and further, depending on the ingredient composition of the waste plastic substrates, there is also a case where a hydrogen chloride gas is generated by the incineration and causes acid rain. Moreover, in cases where plastic substrates are scrapped into soil, there are problems that it is becoming difficult to secure sites for the disposal of the waste, and further, there is also a worry that the waste may reside for a long time without decomposition, so that the environment or the ecosystem in soil may be destroyed.
Also as to coating compositions based on plastic substrates, it is general that coated articles having become unnecessary after use are disposed of by releasing their coating films from base materials and thereafter incinerating the released coating films or scrapping them into soil. Therefore, there are still the above problems.
Thus, bio-based plastics are proposed as substitutes for conventional plastics based on fossil fuels such as petroleum and coals which conventional plastics cause the above problems. For example, bio-based polymers are proposed such as polyhydroxyalkanoic acids, polysuccinic acid alkylenes, and polysaccharides (refer to non-patent document 1 below).
As more specific prior arts, for example, the following arts are proposed.
A coating in which a modified starch is used as a bio-based ingredient is proposed (e.g. refer to patent documents 1 to 3 below). However, the modified starch has defects such that it is brittle and therefore weak to such as bending and easy to scratch. Therefore, the modified starch cannot be used in a wide range, and further is poor in long-time stability such as hydrolysis resistance. In addition, there is a defect that the production cost for carrying out the alkylation of starch is high.
A macromonomer of a polylactic acid and a polymer of this macromonomer are also proposed (refer to patent document 4 below). However, without using a cyclic lactide, such a macromonomer would be difficult to obtain in a high yield. Therefore, the macromonomer becomes expensive similarly to the modified starch.
As a resin composition for coatings, a polylactic acid obtained by copolymerizing lactic acid, a dicarboxylic acid and a glycol is proposed (refer to patent document 5 below). However, by this art, a coating film having a sufficient hydrolysis resistance cannot be obtained.
As other arts using a polylactic acid, there are also proposed such as a polylactic acid resin coating composition having an excellent low-temperature adhesive property (refer to patent document 6 below) and an adhesive for wrapping films which adhesive is excellent in transparency, preservation stability, and adhesion by using a polylactic acid emulsion (refer to patent document 7 below). However, as to these methods, plasticizers or emulsifiers need to be used in amounts larger than conventional, and coating film properties such as hardness and scratch resistance are not sufficient, and further, also as to the problem of the hydrolysis resistance, no solution of such a problem is intended.